The present invention relates to the field of magnetic resonance imaging. It finds particular application in conjunction with radio frequency (r.f.) coils for use therein.
Magnetic resonance imaging (MRI) is a widely used diagnostic imaging method. MRI equipment includes a magnet apparatus for generating a very strong, homogeneous static magnetic field within an examination region. A so-called "open" magnet arrangement includes a pair of pole pieces disposed on opposite sides of the examination region. MRI systems also include an r.f. coil arrangement for exciting and detecting magnetic resonance in the examination region. In order to excite magnetic resonance, transmit coils are used to generate a rotating r.f. field.
A number of sometimes conflicting considerations influence r.f. coil design. In order to maximize the size of the examination region, it is desirable that the coils be as thin as possible. At the same time, it is desirable that the coils be as efficient as possible, so that for a given input power, the coils produce a relatively large r.f. field. It is also necessary that the coils be tuned to the r.f. excitation frequency, for example the Larmor frequency of hydrogen atoms influenced by the static magnetic field. In order to generate an r.f. field which rotates in a plane perpendicular to the main magnetic field, two r.f. coils rotated 90 degrees with respect to each other have been provided, with the coils driven in quadrature.
R.f. coils for so-called open magnet systems have included planar butterfly coils. Each half of a butterfly coil includes a relatively small number of conductor turns (e.g., two) configured to produce the desired r.f. field within the examination region. The coils are connected by circuit traces to a "bank" of discrete tuning capacitors. This arrangement has various drawbacks. First, there tends to be significant capacitive coupling between the rotated coils. This, together with the high voltage present on the coils, has a deleterious effect on coil efficiency. In addition, the circuit bridges associated with the discrete tuning capacitors tend to be narrow, thereby increasing stored magnetic energy which does not contribute to the useful magnetic field. Further, the tuning capacitors themselves must be of relatively high quality and also tend to be bulky. While a larger number of coil turns is desirable, increasing the number of turns tended to decrease the self-resonant frequency of the coil so as to be below the Larmor frequency. Radial current components associated with the coils have also led to undesirable components in the r.f. field.
A pair of coils has been associated with the upper pole piece and a corresponding pair associated with the lower pole piece. The coils nearest to the pole pieces have been oriented in a first angular position, and the coils nearest to the imaging region have been oriented at a second angular position rotated about the z-axis offset ninety degrees from the first. The coils nearest the pole piece have been driven to produce a first component of the rotating r.f. field, while the coils nearest the imaging region have been driven to produce a ninety degree offset field component. However, the efficiency of the coil pair nearer the pole pieces (and hence the r.f. screen or shield) has been lower than that of the pair nearer the examination region. As a consequence, the r.f. field at the center of the imaging region has had a undesirable linear component resulting from the difference in amplitude of the zero and ninety degree components.
The method and apparatus disclosed herein address these drawbacks, and others.